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The name is based on that of a forced induction air compressor which makes a car go faster (a turbocharger).
The turbo button usually accomplishes this by either adjusting the CPU clock speed directly, or by turning off the processor's cache, forcing it to rely on the significantly slower main memory for memory accesses. The button was generally present on older systems, and was designed to allow the user to play older games that depended on processor speed for their timing.
Some systems also supported keyboard combinations Ctrl-Alt-+ and Ctrl-Alt-- for switching turbo mode on and off; ITT Xtra used Ctrl-Alt-\ to toggle.
Calling it a "turbo" button when its function slows the system down can be a bit misleading, but the button was usually set up so the system would be at full speed when the button was "on". The turbo button was often linked to a MHz LED display on the system case, or to a "hi"/"lo" LED display.
While the implementation of the turbo button by manufacturers has disappeared, software developers have compensated with software replacements. One example is DOSBox, which offers full turbo button functionality with adjustable clock speed. Modern PCs that support ACPI power management may provide software controls to switch ACPI performance states or other CPU throttling modes.
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